1. Field of the Invention
The present invention relates to an illumination optical system, and more specifically, it relates to an illumination optical system that illuminates a surface to be illuminated using a plurality of lamp units.
2. Description of the Related Art
Some of illumination optical systems employed in a current projection display apparatus illuminate a surface to be illuminated using a plurality of lamp units in order to illuminate it more brightly. In such a projection display apparatus, a technique is used that combines light beams emitted from a plurality of lamp units in order to reduce the light beam diameter. The light quantity distribution of light emitted from a general lamp unit is not uniform. Therefore, when combining light beams emitted from a plurality of lamp units, it is important to reduce illuminance unevenness.
PCT Japanese Translation Patent Publication No. 2004-524581 (corresponding to U.S. Pat. No. 6,488,379 B2) discloses a technique for equalizing the intensity distribution of combined light emitted from two lamp units. Specifically, lamps and a mirror are disposed such that the central part of light emitted from the larger lamp unit, where the intensity distribution is low, is supplemented with light emitted from the smaller lamp unit.
However, in the technique disclosed in PCT Japanese Translation Patent Publication No. 2004-524581, the smaller lamp unit uses only a region of the reflector near the optical axis, and therefore the parallelism of emitted light is not good. The reason is that the parallelism of light reflected by a region of the reflector near the optical axis is lower than that of light reflected by the periphery of the reflector. If the parallelism is low, the quantity of light blocked by the downstream optical system increases, and therefore the quantity of light usable for illumination decreases.